(almost) Everyone here agrees that it was a colossal mistake that Eglinton was no made fully grade-separated.
Only if ECLRT is built within the current design paradigm (half tunneled, half surface), and
still runs over capacity, then we will have to count it as a mistake.
Many people here are saying it won't happen, and I tend to believe them. The ridership forecasts are never fully accurate, but it isn't likely that the actual demand will be 200% of the forecast.
If we see the actual peak demand within 10-12 k (still manageable with LRT), but could expect 18-20 k if it was a subway, then it is a matter of opportunity cost. You could either build 1 crosstown subway line, or 1 light rail line on Eglinton and another one say on Lawrence for the same price. Either option would have some pros and some cons.
Shouldn't the new government explore the cost of correcting that mistake?
I would suspect that platform height is already too late to change.
I also suspect it's too late to change the train type, unless some complicated switch can be made by sending these trains to Ottawa, Kitchener, Mississauga, or Hamilton, and buying more suited trains for this line.
With cross-over tracks at Laird, grade-separating the line from Brentcliffe to Kennedy trains could still use the Central portion of the "subway" and this construction would have zero effect on the opening date of the central portion.
I suspect the cost of grade-separation would be in the $1B to $2B range.
$2B would probably buy the cost of grade-separation from Laird to Kennedy, had it been a part of the original design. It would likely cost more now if we had to revisit that section, as some contracts should be already signed.
And, that certainly wouldn't upgrade the capacity of the central section which is in the advanced stages of construction now.
We can debate if choosing subway from the onset was a better option, but changing the design mid-way certainly isn't a good option. On the contrary, it can lead to disastrous results.